The gamma-Al2O3 supported CoNiMo tri-metallic catalysts were synthesized by four different procedures involving the variation in metal impregnation sequence. The catalysts were characterized using XRD, BET, HR-TEM, H-2-TPR, Raman, and CO chemisorption techniques. All catalysts were tested for the hydrotreating of bitumen-derived heavy gas oil in a continuous fixed bed reactor operating at typical industrial conditions of 9.0 MPa, 375-395 degrees C, 1 h(-1) liquid hourly space velocity (LHSV) and gas to oil ratio of 600:1 ml/ml. The hydrotreating activity of tri-metallic catalysts is compared with that of bimetallic NiMo/gamma-Al2O3 and CoMo/gamma-Al2O3 catalysts. It was found that various synthesis procedures have a different impact on metal dispersion. The impregnation of Ni prior to Co results in higher dispersion of molybdenum as seen from XRD. The tri-metallic catalyst Co-NiMO/gamma-Al2O3 (1.5Co 2.0Ni 13.0Mo/gamma-Al2O3) synthesized has shown 8% and 38% higher hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) activity as compared to that shown by bimetallic catalysts. The higher activity shown by tri-metallic CoNiMo catalysts is ascribed to the double promotional effect of Co and Ni and formation of three types of active phases NiMoS, CoMoS and Ni-CoMoS. (C) 2017 Elsevier B.V. All rights reserved.